1. FIELD of the Invention
This invention relates to hydraulic cylinder and piston assemblies. More specifically this invention is concerned with the attachment of a piston to a piston rod, the design of the piston and the apparatus used to provide a seal between the walls of the hydraulic cylinder and the piston.
2. Description of the Prior Art
The usual method of attaching a piston rod to a piston is through the use of a piston pin. The piston pin allows the piston to maintain alignment in the cylinder bore while allowing partial angular displacement of the piston rod.
In hydraulic cylinders piston pin attaching methods are seldom seen. Hydraulic piston and rod assemblies are generally mounted such that the cylinder is pivotally mounted to a frame and the exterior end of the piston rod is pivotally mounted to a reactive member. Therefore it is not essential that the piston be pivotally mounted to the piston rod. This being the case several alternative piston to rod attaching means have been developed.
In hydraulic units it is common to have the inboard end of the piston rod threaded to receive either the piston itself, which would be internally threaded, or a nut positioned after the positioning of the piston on the rod.
Attachment by this method is usually used for pistons having diameters up to about four inches. Cylinders and the matching pistons above a certain diameter may require an alternative attachment means as the torque necessary to assure the attachment of the piston to the piston rod is so high that assembly becomes difficult.
One alternative method of attaching a piston to a piston rod is to use a split ring and a two part piston. In a typical embodiment of this type the piston rod will have a major groove as well as a plurality of minor grooves machined onto the inboard end thereof. The major groove is for receiving a segmented locking ring and the minor grooves provide recesses to accommodate seals.
The two part piston assembly comprises a first piston part equipped with a plurality of threaded apertures and a second piston part having an identical number of smooth bore apertures. The first part of the piston is positioned on the piston rod then the locking ring is positioned such that it resides in the major groove of the piston rod. The second part of the piston is positioned on the piston rod and attaching bolts are passed through the apertures of the second part of the piston, the apertures of the split ring and into the threaded apertures of the first part of the piston. When the attaching bolts are torqued down the piston, now a three part assembly, is held fast to the piston rod by the locking ring.
The number of attaching bolts may be varied according to the capacity of the hydraulic cylinder. Generally larger piston to rod assemblies have more (and larger) attaching bolts then smaller assemblies.
Prior art two part piston assemblies may be equipped with a wear ring on each piston part as well as a seal on the cylinder engaging face of each piston part. In addition other seals on the inner rod engaging bore of each piston part are needed to limit fluid flow between the piston and the rod. These inner seals, which attempt to block flow or leakage between the piston and the rod present sealing problems that are difficult to overcome in high capacity hydraulic cylinders.
The cost of the previously discussed piston to rod assembly is high due to the necessity of extensive machining for piston rod seals and the length and subsequent diameter of the attachment bolts.